The immunochromatographic method is an immunological test method of determining the presence or absence of an analyte substance by making the analyte substance captured by labelled particles move through a porous support in capillary phenomenon, efficiently bringing the labelled particles into contact with a capturing substance locally (e.g., linearly) immobilized on the porous support and thus, concentrating the analyte substance, and making the immobilized line colored by the capturing substance. The immunochromatographic method has the following three characteristics:
(1) Rapid test is possible, as the period needed for test is 20 minutes or less.
(2) Multi-item analysis is possible, because the operation was simple and easy, demanding only dropwise application of the sample.
(3) No special detector is needed and the test is simple and easy, allowing test by general users.
Because of these characteristics, the immunochromatographic method has been applied as a pregnancy test reagent and an influenza test reagent, and attracting attention as a new POCT (Point Of Care Testing) method.
The POCT is a test for diagnosis that is carried out at a place as close to the patient as possible. Conventionally, samples collected such as blood, urine and infected organs were sent to the central laboratory of hospital or a professional laboratory for analysis for data, and thus, a long period (e.g., at least one day) was needed before final diagnosis. The POCT above promises rapid and accurate therapy, based on test information obtained instantaneously and allows emergency tests in hospital and during operation, and thus, there is an increased need for it recently in the medical settings.
Currently, gold nanoparticles are used most frequently as the labelled substance in the immunochromatographic method. The gold nanoparticles have a large absorbance and are thus superior in visibility, but have only a single color, consequently causing a problem of restriction of the color for use. In addition, gold particles having a larger particle diameter often aggregate and blacken over time, causing a problem that it was difficult to make the gold particles develop a clear color.